Have you considered some of the landlord's actions might not have been illegal? Anyone from the UK that can chime in on this? Is there an authority responsible for this?
35. Landlord's Covenant for Quiet Enjoyment
The Landlord covenants with the Tenant, that, so long as the Tenant pays the rents reserved by and complies with the obligations of this lease, the Tenant shall have quiet enjoyment of the Property without any interruption by the Landlord or any person claiming under the Landlord except as otherwise permitted by this lease.
There are clauses covering emergency entry for repairs (e.g. burst water pipes, leaking gas) in short-term rental agreements but - as others have said - they require "reasonable" notice in almost all cases aside from dire emergency, and the tenant can still refuse entry.
It could still have been illegal, but it was definitely in the lease.
If they turn up unannounced this is actually harassment under UK law.
We had some rather unpleasant slum landlords between the 1950s and 1970s which caused a few laws to be introduced. Unfortunately the nature of being a landlord seems to attract certain people who find new and creative ways to be dicks. Not the majority of landlords I will say who are mostly pretty good, but enough to cause problems.
They probably have to do this repeatedly to meet the threshold for harassment.